Device for supplying a suspension to a filter at different levels within the filter and in proportions such that a uniformly distributed flow of suspension is achieved across the filter area

ABSTRACT

A suspension is delivered to a filter bed comprised of granular filter medium in a continuously working filter at the lower part of the bed and is then permitted to flow upwards through the bed. With the intention of optimizing the capacity of the filter, the suspension is now guided into a central region of the bed. The suspension is divided from this region into at least two specific proportional quantities which are each led from the filter bed in a respective ring-shaped pattern at mutually different heights in relation to the extension of the bed, these ring-shaped patterns having different diameters and each being adapted to the extension of the filter medium in the direction of the diameter.

BACKGROUND OF THE INVENTION

The present invention relates to methods and devices for achieving auniformly distributed flow of suspension through a granular filter bedin a continuously working filter. The invention therewith achievesoptimization of the capacity of the filter.

By a continuously working granular filter is meant here and in thefollowing any type of filter in which filter medium is removedcontinuously from the bottom of the filter bed, washed and then returnedto the upper side of the filter bed as suspension passes through thefilter bed and cleaned thereby. An example of one such filter isdescribed in Swedish Patent Specification 7602999-0. The particulatefilter medium, normally sand, is enclosed in a circular or polygonalcontainer which has a conical or pyramidal bottom. The suspension oremulsion to be treated is taken in through an inlet at the bottom of thefilter bed. The suspension or emulsion flows into the filter bed througha number of pipes whose orifices are located beneath a roof or ceilingstructure. The orifice of a lifting device, conveniently a mammoth pumpor air lift, is placed in the lowermost part of the conical bottom. Sucha pump uses air as the transporting medium. The pump pipe or tubeextends up through the filter bed to a washing device, from which washedsand is distributed over the upper side of the filter bed. The incomingsuspension thus flows in countercurrent to the filter medium outwardsthrough the filter bed towards progressively more cleaner filter mediumand leaves the filter through an outlet, via a weir means. A cone ispositioned in the lower part of the filter bed with the apex of the conefacing upwards, so as to contribute to a desired flow profile of thefilter medium. This cone is referred to in the following as a guide one.

The known technique includes several solutions to the problem ofintroducing the suspension into the filter bed in a manner which willresult in a uniformly distributed flow over the filter area. Known tothe art are devices which distribute the suspension in differentdirections out towards the periphery with the aid of radially extendingarms. Other inlets take place around the outer wall and also further intowards the center. A common feature of all solutions is an assumptionthat precisely this or that solution will provide a uniformlydistributed flow throughout the whole of the filter. This assumptionremains hypothetical unless it can be proved to be true with the aid ofconcrete measurements. The difficulties in taking such measurements haveevidently presented such a large obstacle to obtaining a true picture ofprevailing realities that the matter has quite simply been ignored.

SUMMARY OF THE INVENTION

The present invention is based on a large number of measurements of theflow conditions that were taken at a plurality of measuring pointsdistributed both radially and axially in the filter bed. The result ofthese measurements was surprising. Two very essential observations couldbe made. Firstly, it was found that the flows had a very strong tendencyto curve out towards the outer surface of the filter immediatelydownstream of the inlet. Those inlet devices that were placed adjacentthe outer surface and also slightly inwards of the outer surfaceproduced an equally as large flow within the 30% of the total area thatlay proximal to the outer surface as in the whole of the remainder ofthe filter. In hydro-mechanical terms, it is possible that this was dueto a smaller grain density of the filter material around the outersurface or mantle of the filter bed. Secondly, it was found that theflatness of the upper surface of the filter bed had a strong influenceon the flows in an area located some decimeters beneath the surface. Theupper surface of the filter bed had an undulating configuration,presenting ridges or crests, and the liquid flows deviated beneath partsof the upper surface and were directed towards the nearest troughs ofthe undulations. The important conclusion that can be drawn from theseobservations is that the filter bed was not utilized to an optimum andthat there was much to gain from an optimally utilized filter bed. Onecharacteristic feature of an optimally utilized filter bed is that eachsmall part of the filter bed is utilized to a maximum for filteringpurposes irrespective of where this part is located in the filter bed,i.e. that equally large suspension flows will pass through the bed. Whensuch is the case, it is possible to increase the total flow through thefilter to a higher level without lowering the extent to which thesuspension is cleaned. Alternatively, the amount of filter medium can bedecreased, resulting in a lower total height, construction height, ofthe filter.

A large number of measurements taken in filters having varyingsuspension inlet conditions and with varying suspensions right down toclear solutions have given conditions for one such optimally utilizedfilter and have led to the present invention. A major part of thesuspension flow shall be led into the filter bed where the filter areais momentarily the smallest, this area, at the same time, being the areain which the fall rate of the filter medium is the greatest. One sucharea is found at the lower edge of the guide cone, this edge normallybeing located in that region where the beginning of a bottom cone isfound. It has been found that a uniformly distributed flow can beachieved in the outer region of the outer surface by allowing thesuspension to flow-in around the edge of the guide cone.

However, it was also found that it was not possible to allow the wholeof the suspension to flow-in around the bottom edge of the guide cone,since the surface load in the confined filter area then becomes toohigh, causing suspended particles to be torn loose from the sand grains.Neither could a uniformly distributed flow be obtained over the whole ofthe filter area or completely up through the whole of the filter. It wasfound necessary to also introduce part of the suspension very centrallyto the filter bed. This part of the suspension quantity enters at alater stage in the filter bed and fills-out that area of the filter thathas not been filled satisfactorily from the lower edge of the guidecone. The uniformly distributed flow can be obtained over the whole ofthe filter area and through the major part of the filter, by adaptingthe distribution of suspension to the two inlet regions. The inventionhas obtained the characteristic features set forth in the followingclaims against the background of the aforesaid.

According to one refined embodiment of the invention for optimalutilization of the whole of the filter the upper surface of the filterbed is totally flat.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to theaccompanying drawing, the single figure of which illustratesschematically the lower part of the continuously working filter inaccordance with the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

The drawing illustrates schematically how the introduction of asuspension to the filter bed can be achieved in accordance with theinvention. The lower part of a filter bed 1 is enclosed by means of apreferably circular shell or mantle 2 and a bottom cone 3. The transportpipe 4 of an air pump, extends centrally through the filter bed andextends up to a filter medium washing device (not shown) and to an airdelivery pipe 5 which delivers air to the pump. A suspension deliverypipe 6 extends concentrically around the pump and exits into the upperpart of the guide cone 7. The guide cone 7 has an open bottom and partof the suspension is intended to flow out into the filter bed around thelower edge 8 of the cone and into the confined region 9 between theguide cone 7 and the bottom cone 3. A number of holes 10 are disposedaround the upper part of the guide cone in a circular path, so that thesuspension is able to flow relatively centrally into the filter bed 1.In the illustrated case, the holes 10 are shielded from the filtermedium by means of a conical collar 11 placed immediately outside theholes. In the illustrated case, suspension flows into the filter bedaround the lower edge of the collar 11. In another embodiment accordingto the invention, each hole is shielded from the filter medium by meansof an outwardly lying cover or lid. The holes may also be placed aroundcircular paths on different cone heights. The flow obstacle or barrier12 is provided below the holes 10 within the guide cone. These barriersare intended to retard the powerful kinetic energy that is imparted tothe suspension through the vertical infeed channel 6, and also todistribute the suspension volumes to the lower part of the guide coneand to the holes in the upper part of said cone respectively, in desiredproportions. The flow barrier 12 may conveniently have the form of ahorizontal plate attached to the guide cone and provided with aplurality of holes. In this case, the holes are suitably placed so thatthe suspension entering from the pipe 6 will not impinge on the holes,but will be retarded.

In the case of another embodiment according to the invention it is notnecessary for the more central infeed of suspension to take placethrough holes provided in the guide cone, and the holes may be placeddirectly on the suspension delivery pipe 6 immediately above the upperpart of the guide cone. It is also necessary in the case of thisembodiment to provide suspension flow barriers either in the pipe or inthe upper part of the guide cone. It is not necessary, on the otherhand, to shield these holes with cover plates or collars.

In order to obtain a uniform flow into the filter bed around the loweredge of the guide cone and also at the edge of the upper collar it isnecessary for the edges to be orientated horizontally and to be rigidlyheld in this position. In order to facilitate an even flow of suspensioninto the filter bed, the aforesaid lower edge is conveniently providedwith notches in some form or another, for instance in the form oftriangular recesses, serrations, not shown.

Notwithstanding the composition of the suspension, it has been foundsuitable to permit more than 65% of the suspension, but not more than80% thereof, to be led into the filter bed around the lower edge 8 ofthe guide cone and the remainder of the suspension relatively centrallythrough the holes 10 provided in the upper part of the guide cone. Inaddition, the bottom edge of the guide cone will preferably be spacedfrom the vertical defining surface of the filter or an imaginaryextension of the filter by a distance which constitutes at least onethird of a radius within the filter medium, and the inner edge beneaththe collars or like devices will preferably be spaced from the innersurface of the filter medium by a distance of less than 1/3 of theradius within the filter medium, i.e. in the case of the illustratedembodiment against the infeed pipe.

Devices for obtaining a flat upper surface may have many differentconfigurations. The devices may have the form of expensive rotaryscrapers. This should be weighed against the gain afforded by a simplerdevice which provides an essentially flat upper surface with acceptableslight undulations in the filter medium. A compromise may be found in adistributor cone which is located beneath the washing device and whichhas the appearance of a star with a large number of jets radiating outtowards the periphery.

The economic gain achieved with a filter which is utilized to an optimumis a palpable gain under all circumstances. The filters will either havea lower structural height already in the project stage and can therewithbe accommodated in buildings of smaller heights or can be accommodatedin existing buildings, or the number of filters can be made to be fewerin the case of large plants, since a larger volume of suspension can beallowed to pass through each filter.

We claim:
 1. A device for introducing a suspension into a filter bedhaving a granular filter medium in a continuously working filter thedevice comprising: a centrally located and downwardly open guide conethat embraces a lower part of the filter bed to ensure that the filtermedium will obtain a desired flow pattern, the device configured todeliver suspension to an interior of the guide cone in its top regionand downwards so that the suspension is led out to the filter bed fromthe interior of the guide cone and also along the bottom edge of theguide cone at an at least one level above said bottom edge in order toobtain a uniformly distributed flow of suspension across the filterarea, said device having infeed pipes which open into a lower part ofthe filter bed where said downwardly open guide cone is placed centrallywith the apex of the cone pointing upwards so as to ensure that thefilter medium will obtain the desired flow pattern, wherein an orificeof the infeed pipe lies within the top region of the guide cone, saiddevice further having outlets to the surrounding filter bed which arecomprised of a number of openings through a mantle surface of the guidecone located in rows at certain heights on the cone and/or openingsthrough the infeed pipe above the guide cone and passageways along thebottom edge of the guide cone,wherein the openings through the mantlesurface of the guide cone are shielded by circumferentially extendingcollar means which is spaced slightly from said openings through themantle surface and seals tightly against the outer surface of the guidecone above the openings through the mantle surface.
 2. The deviceaccording to claim 1, wherein said device is arranged so that the volumeof suspension led out at the highest level comprises at least 20% of thetotal flow, and so that the volume of suspension led out at thelowermost level comprises at least 65% of the total flow.
 3. The deviceaccording to claim 1, wherein the openings through the mantle surfaceare disposed along a circular path around the mantle surface of theguide cone.
 4. The device according to claim 1, wherein a center axis ofthe orifice of the infeed pipe coincides with an axis of the guide cone.5. The device according to claim 1, further comprising a suspension flowbarrier located immediately beneath the openings through the mantlesurface and which functions to distribute the suspension to thedifferent outlet levels in desired proportions.
 6. The device accordingto claim 3, wherein the openings through the mantle surface are disposedalong several circular paths at different heights along the guide cone.7. The device according to claim 1, wherein the bottom edge of the guidecone is spaced from an outer cylindrical defining surface of the filterbed or from an imaginary extension of the defining surface by a distanceequal to at least one third of the radius of the outer cylindricaldefining surface.
 8. The device according to claim 1, wherein theopenings through the mantle surface are spaced from an inner cylindricaldefining surface of the filter bed by a distance which is less than onethird of the radial extension of the filter bed.